The present invention generally relates to offshore platform assemblies, such as self-elevating marine vessels known as jack-up platforms which are adapted to be deployed in a body of water. More particularly, the present invention relates to a platform assembly with inclined, movable legs which can be lowered to the sea bottom to support the platform over a body of water and which are raised for transit of the platform.
Jack-up floating platforms are the most common type of movable offshore drilling units. Such units were first used in the 1950's and today account for approximately 500 such vessels in the worldwide service. As a general rule, these rigs have three or more legs, which are perpendicular to the hull and which are jacked down to the sea floor and are fixed at the sea floor, after which time the platform hulls are lifted above the wave action of the seas.
Another type of jack-up rigs or platforms utilizes inclined, or slanted legs.
The platforms of the second type are deployed in deep waters (250 feet and greater) and therefore their legs have a relatively greater length than the platforms in shallow waters. Therefore, the legs are often positioned outward from the platform at a small angle, for example 1 to 10 degrees, or even more, from the vertical, so as to provide a larger foundation area for the erected platform. The platform, when in installed position, has a symmetrical arrangement and affords considerable rigidity and resistance to overturning, bending forces caused by wind, wave and current.
The operational experience with this type of rig was very good, once they had been jacked-up on a drilling location. However, the tilting mechanism proved to be a source of great difficulty during jacking operations. Such factors as safety, high maintenance costs and high initial costs proved prohibitive in manufacturing and deploying more than a few of such platforms worldwide. The tilting mechanisms employed in the past are massive pieces of machinery and structure. While a few such units still exist, non have been built since 1976 because of high cost and poor safety considerations.
A special concern was caused by the bending moment on the legs while the platform was being erected above the sea level, since rigid guides within which the legs were received, exerted considerable bending loads and shear forces into the legs. Since the leg bottoms are fixed at the sea bottom while the hull is afloat and during the hull lifting action, the legs support the hull weight as the hull is elevated to its desired drilling height and the leg guides gradually impose increasing bending loads in the legs and increasing reaction forces in the guides, most particularly in the lower guides. Such forces induce high stress levels in legs and guides even to the degree that the structure may fail.
The present invention contemplates provision of a jack-up platform having inclined legs, wherein the bending loads and shear forces on the supporting legs are considerably reduced and minimized, thus eliminating the major drawback associated with the prior art. Tilting structures and mechanisms are not employed in this invention.